Pulsed bias current for gain switched semiconductor lasers for amplified spontaneous emission reduction

Abstract

Gain switched laser diode pulses are used as seed pulses for optical pulse generation. ASE is reduced by applying a prebias to the laser diodes at an amplitude less than that associated with a laser diode threshold. An electrical seed pulse having an amplitude larger than that associated with laser threshold is applied within about 10-100 ns of the prebias pulse. The resulting laser diode pulse can be amplified in a pumped, rare earth doped optical fiber, with reduced ASE.

We claim: 
     
       1. A seed laser source, comprising:
 a laser diode; and 
 a laser diode driver configured to apply a pulsed laser drive signal to the laser diode, wherein the laser drive signal is coupled to provide:
 a seed signal comprising a first seed pulse to the laser diode, the first seed pulse having an amplitude greater than a laser diode threshold amplitude, and 
 a corresponding prebias signal comprising a first prebias pulse having a duration in excess of 2 ns and an amplitude less than the laser diode threshold throughout the duration, and that is applied within about 50 ns of the first seed pulse, and wherein: 
 the amplitude of the first seed pulse is at least 10 times the laser diode threshold amplitude, and 
 the laser diode driver is configured so that the first prebias pulse terminates prior to the first seed pulse, such that the first seed pulse is not applied during the duration of the first prebias pulse. 
 
 
     
     
       2. The seed laser source of  claim 1 , wherein a product of an effective amplitude of the first seed pulse and a duration of the first seed pulse is at least 2 times greater than a product of an effective amplitude and the duration of the first prebias pulse. 
     
     
       3. The seed laser source of  claim 2 , wherein a product of an effective amplitude of the first seed pulse and a duration of the first seed pulse is at least 20 times greater than a product of an effective amplitude and the duration of the first prebias pulse. 
     
     
       4. The seed laser source of  claim 1 , wherein the first prebias pulse has an amplitude less than about 0.5 times the laser diode threshold and is applied within about 25 ns of the first seed pulse. 
     
     
       5. The seed laser source of  claim 1 , wherein the first prebias pulse has an amplitude less than about 0.2 times the laser diode threshold and is applied within about 25 ns of the first seed pulse. 
     
     
       6. The seed laser source of  claim 1 , wherein the laser diode driver includes a seed signal generator and a prebias signal generator, and a signal combiner configured to combine the seed signal and the prebias signal and couple the combined signal to the laser diode. 
     
     
       7. The seed laser source of  claim 1 , wherein the laser diode driver is configured to apply the pulsed laser drive signal to the laser diode at a repetition rate between 1 kHz and 100 MHz. 
     
     
       8. A method, comprising:
 establishing optical gain in a laser gain medium; 
 producing an optical seed pulse by applying a prebias pulse signal to a laser diode, and a corresponding seed pulse signal to the prebiased laser diode, wherein the prebias pulse signal has a duration of at least 100 ns, and an amplitude that is less than 0.5 times a laser diode threshold throughout the duration and terminates prior to the application of the corresponding seed pulse signal, such that the seed pulse signal is not applied during the duration of any preceding prebias pulse signal; and 
 producing at least one optical pulse from the laser gain medium in response to the optical seed pulse. 
 
     
     
       9. The method of  claim 8 , wherein the laser gain medium is a doped optical fiber, and further comprising situating the optical gain medium in a laser cavity. 
     
     
       10. The method of  claim 9 , wherein the laser cavity is defined by one or more fiber Bragg gratings or fiber couplers or combinations thereof. 
     
     
       11. The method of  claim 8 , wherein the seed pulse signal has an amplitude that is at least 5 times a laser diode threshold. 
     
     
       12. The method of  claim 11 , wherein the prebias pulse signal has an amplitude that is less than about 0.2 times the laser diode threshold. 
     
     
       13. The method of  claim 12 , wherein the seed pulse signal is applied within 10 times a laser diode gain lifetime after the prebias pulse signal. 
     
     
       14. The method of  claim 12 , wherein the seed pulse signal is applied within about 5 times a laser diode gain lifetime after the prebias pulse signal. 
     
     
       15. A system for producing optical pulses, comprising:
 a laser diode; 
 a laser diode drive source configured to repetitively apply a prebias pulse signal and a corresponding seed pulse signal to the laser diode to produce a seed laser pulse, wherein the prebias pulse signal has a duration in excess of 2 ns and an amplitude that is less than a laser diode threshold throughout the duration, and wherein the prebias pulse signal terminates prior to application of the corresponding seed pulse signal, such that the seed pulse signal is not applied during the duration of any preceding prebias pulse signal; 
 a doped optical fiber situated to receive the seed laser pulse; and 
 at least one pump laser configured to couple a pump laser beam to the doped optical fiber so as to produce optical gain in the doped optical fiber. 
 
     
     
       16. The system of  claim 15 , wherein the prebias pulse signal and the seed pulse signal have amplitudes associated with about 0.5 times or less of a laser threshold and at least about 4 times the laser threshold, respectively, wherein the prebias pulse signal is applied within about 25 ns of the seed pulse signal. 
     
     
       17. The system of  claim 16 , wherein a product of an effective amplitude of the seed pulse signal and a duration of the seed pulse signal is at least 2 times greater than a product of an effective amplitude and the duration of the prebias pulse signal. 
     
     
       18. A method, comprising:
 repetitively applying a seed signal to a seed laser diode so as to produce seed pulses; 
 amplifying the seed pulses with a fiber amplifier; 
 repetitively applying a prebias pulse signal to the seed laser diode prior to application of the seed signals, wherein a duration and amplitude of the prebias pulse signal are selected so as to control amplified spontaneous emission, wherein the prebias pulse signal is modulated between zero and an amplitude that is less than a seed laser diode threshold, and wherein each application of the prebias pulse signal terminates prior to the next application of the seed signal such that a given seed pulse is not produced during the duration of a prior application of the prebias pulse signal. 
 
     
     
       19. The method of  claim 18 , wherein the modulated prebias pulse signal has a duration of at least 100 ns, and an amplitude that is less than 0.5 times a laser diode threshold throughout the duration.